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Building an Integrating Sphere ...

There are already articles on CPF about different light measurement designs, but none that I found on Integrating Sphere construction. For me, finding a hollow, white sphere was the first challenge. After quite a few searches, a 12" hollow styrene sphere was found at Barnard (phone 888-584-3637). Price with shipping was about $38 - the shipping seemed high until the sphere arrived in a carton almost twice as big as the item.

A top and bottom plate were cut from 1/2" plywood. The sphere measured exactly 12" diameter, and the plywood was cut to 11 7/8" (square) to give the sphere a small amount of compression when assembled.

Four posts (also 11 7/8") were cut to connect the upper & lower plates, and the posts and plates were assembled with yellow glue & brad nails.

The sphere was placed inside the assembly, and four side panels were cut from 1/8" plywood. These are attached with yellow glue & brad nails.

The upper hole is cut with a holesaw to match the diameter of the light meter sensor. The light port is also cut with a holesaw to fit the light tube.

A piece of 2 3/4" stainless tube is used here, but PVC pipe would work just as well. The light tube was coated with Satellite City Special T Super Glue, the inside of the hole was misted with accelerator, and the tube was inserted & twisted until the glue set (about 5 seconds)

Except for applying a finish to the wood, the IS is complete. The light meter is an Extech EA31 (range to 20,000 Lux). It was found on Amazon for under $100 & gives repeatable readings. Since the IS is a comparative device, my concern is that the readings are the same every time a light is retested.

Re: Building an Integrating Sphere ...

very nice construction job. kudos to you for your fine effort and excellent fabrication.

based upon some of the comparative readings, not sure if the setup is truly integrating. obviously, the meter is still reading in Lux. that said, in some respects it still seems to be reading mainly "hotspot" (Lux) readings (plus some, perhaps???) and may not be truly integrating. why would i say this? well,...

unless i have my head screwed on totally backwards, just compare two or three of the SF lights. SF rates the E1B at, IIRC, 80lm, the U2 and L4 at 100lm. now, the lux (hotspot) readings of these three lights are quite different, i would imagine, based upon hotspot (E1B > U2 > L4). your readings are ordered in accordance with what i would imagine it would be if only HOTSPOT was considered (again, E1B > U2 > L4).

your readings indicate, comparatively, for those three SF lights what one might expect from Lux/hotspot readings and not what SF claims for Lumen ratings.

i would like to know what the Lux readings are at a straight line linear distance equivalent to the diameter of the sphere. how different are they from the Lux readings using the sphere? how do they compare to each other?

all this is *NOT* said to denigrate in any way either your exemplary initiative, nor your fine efforts. i just have these questions based upon the readings obtained and published in your Post.

Re: Building an Integrating Sphere ...

it still seems to be reading mainly "hotspot"

I understand your thoughts, and agree that some of the readings are not what I expected. Perhaps 12" is too small for full integration of the reflected light. However, the diffusion dome of the meter is looking straight down at the bottom of the sphere, where it sees no direct light. The hotspot from the light is projected against the sphere wall opposite the light port.

I may need to fabricate & install a baffle at the bottom of the sensor dome port. This would lower all readings by an equal amount & eliminate any overreading from a hotspot.

FWIW, Chevrofreak used essentially the same locations for both light port & sensor port:

Re: Building an Integrating Sphere ...

understood about the relative positions of the light source and sensor. just having the right angle relationship between source and sensor does NOT guarantee that proper integration is occurring. one could use one flat mirror to reflect light at a right angle from source to sensor. this obviously would not be integration, and the meter would only read the hotspot/highest/brightest portion of the reflected light. i'd like to try to determine how much (in a purely gross qualitative sense; not a quantitative sense) integration is really taking place in your sphere.

it still seems to be reading the reflected hotspot plus possibly some non-hotspot reflected light, and NOT integrating more/"all" of the light output. if it did the U2 and L4 ought to read higher than the E1B (again, unless i have my head screwed on backwards).

if you measure the Lux output straight on at 12" (the diameter of your sphere) without the sphere, there are only three relative possibilites. the readings of each will be higher, lower, or the same when compared to the "in sphere" readings.

if higher, then your sphere [readings] and Lux meter are only measuring SOME of the reflected light which appears to the meter to be reduced by the reflection in the sphere.

if lower [, but still the same relative relationship E1B > U2 > L4], then there is some integrating taking place, but probably (again, based upon U2 and L4 being SF rated at 100lm vs. E1B rated at 80lm) still to a degree only measuring the brightest portions of the beam output.

if lower, but a different relationship (e.g. E1B, NOT being the highest reader), then there may be something amiss in the straight line Lux readings as one would expect the E1B to be the brightest (i really don't know how they compare in Lux testing; i'm just guessing here based upon the brightspot of each light - i own all three of these SF lights).

if the same (i don't expect this to be the case), then no integration is taking place.

this is the values of performing a "sanity" check by taking normal straight line/linear Lux readings. i suggest using the very close 12" distance since that is the diameter of your sphere, or use 24" (or any other distance - easier to use a multiple of the sphere's diameter - makes purely mental calculation of the inverse square easier; i can't easily do inverse square with decimals in my head) and use the inverse square law of radiating energy to compare the results to the sphere's Lux reading.

again, excellent work on the concept and fabrication. obviously, a valuable original Thread and Post on your part.

Re: Building an Integrating Sphere ...

I'll start by saying I know nothing about this subject but I would have thought the best position for the light sensor would be close to the input port, if the tube at the input was set an inch or two into the sphere it would also act as a baffle.
Norm

Re: Building an Integrating Sphere ...

Very interesting work indeed, Can you see white light coming through the sphere to the outside world? Is it glowing? If so you are losing light because the material is not as reflective as it should be. What ever amount of light is coming out is not being internally reflected.

Half-watt's test sounds valid as a way to determine if its integrating or just getting most of the hot spot readings.

Re: Building an Integrating Sphere ...

Re: Building an Integrating Sphere ...

I looked again in the sensor port (which is 1/2" plywood followed by 1" of styrene foam) and saw that the styrene foam is capturing a some of the spillbeam, leading to an inflated reading on some lights.

Since the sensor port is 2" diameter x 1 1/2" thickness (to the inside of the sphere), a piece of 2" tubing was painted flat black inside, and pushed into the port. The new readings are as follows:

Malkoff M60 in Surefire 6P host - 2,880

Surefire E1B on high setting - 1,780

Surefire U2 on high setting - 1,774

Novatac 120P on high setting - 1,582

Muyshondt Nautilus high setting - 1,370

Surefire L4 - 1,203

Malkoff M60LL in Surefire G2 - 935

Surefire E2L (single stage Cree) - 864

McGizmo Sundrop - 727

Surefire E1L (single stage Cree) - 614

Muyshondt CR2 Ion - 454

If you look at these numbers as a percentage, they are nearly identical to the first test without the black tube baffle. The numbers below are in percent, with the Malkoff M60 used as a 100% standard (numbers are test 1/test 2)

Malkoff M60 in Surefire 6P host - 100/100

Surefire E1B on high setting - 63/62

Surefire U2 on high setting - 61/61

Novatac 120P on high setting - 56/55

Muyshondt Nautilus high setting - 47/48

Surefire L4 - 37/42

Malkoff M60LL in Surefire G2 - 32/32

Surefire E2L (single stage Cree) - 29/30

McGizmo Sundrop - 25/25

Surefire E1L (single stage Cree) - 21/21

Muyshondt CR2 Ion - 15/16

To me, the only reading that falls outside the expected lumen curve is the E1B. I bought a few of these and sold all but this one (which is on a McGizmo Ti McClicky Pak host, not the standard SF body). It has about the same low setting as any other E1B, but the high is the brightest of any that I've seen. Maybe SF put the wrong driver board in this head, maybe the emitter bin is at the top of the XR-E series, but without a doubt it is brighter than the U2 on a ten foot, white wall bounce.

Re: Building an Integrating Sphere ...

regarding your E1B observations: understood. seen this type of thing before (though mine was a Fenix P1D-CE which was brighter than four P1D-Q5's; the fifth P1D-Q5 was brighter though). it's probably the LED and *NOT* the E1B electronics responsible for what you are seeing. the disparity b/t U2 and L4 readings though still indicates more of a Lux type of reading rather than a Lumen type of reading, if i'm not mistaken (and feel free anyone to correct me if you feel that i am mistaken). however, this assumes that SF has correctly rated both the U2 and L4. SF appears to rate conservatively and perhaps they are both >100lm, but that the U2 is more conservatively rated??? don't know. also, only one specimen of each light is being used, so perhaps the U2, in this case, has more overall light o.p. than the particular L4 specimen being tested???

also, thinkin' a bit more, i think my early contention that you would get a higher Lux reading in the sphere vs. linear is just plain wrong. since the sphere diffuses the light, including the hotspot. this ought to even out the brightness. looks like i did have my head screwed on backwards - wouldn't be the first time!!!

Re: Building an Integrating Sphere ...

I would have thought the best position for the light sensor would be close to the input port

The sensor can be located in a number of positions, so long as the sensor does not see either the light source or the hotspot. The easiest sensor position (for a closed sphere) is 90° to the source. Larger spheres are split in half and hinged - this allows the source to be located in the exact middle of the sphere, with a baffle blocking direct radiation to the sensor.

Can you see white light coming through the sphere

The foam is 1" thick, so none of my lights (even the 240 l Malkoff) penetrates.

Re: Building an Integrating Sphere ...

I understand your concern about the baffle, and the need for no direct light to strike the sensor dome, or strike inside the sensor port where it would reflect into the sensor dome.

I did conduct a test for this. All room lights were turned off. The photosensor was removed from the top port and a flashlight was turned on & placed in the light port. I inspected the sensor port to make sure that no direct light was hitting the inside of the sensor port tube. In a dark room, the only light I saw was the light at the bottom of the sphere. For this reason, I believe that the meter is reading an integration of total luminous flux.

A calibration light source is something I'd like to find. The commercial ones plug into a wall outlet, and the output varies with the size of the sphere. That way, the relative readings could be converted, using a conversion factor, to lumens.

Re: Building an Integrating Sphere ...

I looked inside the IS we use at work today and also talked to our head PhD Optical Engineer about this. The Baffle is very important in not having false high readings. Its a small circular 2 dimensional object that is literally directly between the 2 ports. Its tilted up in the circular cavity so that any light directly from the input port would cast a circular shadow on the meter port. Since its on an curving slope the best way to describe is that it is parallel to the flat vertical opening surface for the meter port. It is the same flat white color/texture as the rest of the sphere.

In this way lights with a wide angle head cannot have any direct light impingement in the meter port.

You may not think that the lights are going directly to the port but the results you posted may have a false high reading. There should be nothing that sticks into the inside surface from the outside world that goes beyond the wall thickness.

You have gone this far, might as well try it, then divide all your readings by 36 (radius squared) to see how close they get to the rated lumens of the lights you use. This may be an over simplification, but its quick and easy for now. May need a fudge factor. The sensor in the one at work is only about 1.8" diameter opening.

I notice on the one I keep testing with at work (its a 6 inch sphere by the way and its calibrated with the meter and our engineers trust the readings of it for all kinds of LED light sources), that if I move the light into the sphere at all the readings actually go down. It appears the best readings are with the light source directly at but not past the opening of the sphere. That is why I believe that the black tube at the inlet port is really going to affect the readings negatively. If you are going for all comparative readings, it still doesn't buy anything to have that black tube on the inlet. Real IS's have no such thing.

I would expect all the readings to go down some what with the baffle installed.

In theory a 235 lumen light source of white light should read 8460 lux if the baffle does its job right.

If I am off on these numbers it means I should have gone to bed already. Good luck.

Re: Building an Integrating Sphere ...

it still doesn't buy anything to have that black tube on the inlet. Real IS's have no such thing.

Real IS's aren't made with hollow stryofoam balls & cost just a little more than $38 ... and the light/lamp/LED being tested is typically located centrally in the sphere, separated from the photo detector by a baffle that prevents direct light readings. I couldn't figure a way to easily construct such a sphere, which involves cutting the sphere in half to produce two hemispheres hinged together.

The baffle probably should be installed, as it is common to most IS designs available, including the simple design I used:

In the example above, there is a baffle both above & below the light port (entrance aperture). The dual baffle arrangement is most likely used to prevent a wide angle light source (like a McGizmo Mule or Sundrop) from directly striking either the photodector or the bottom of the sphere - both or either of which would give an inflated reading.

the results you posted may have a false high reading

I also believe they did. The photo detector was looking through the 1" styrofoam sphere wall. That's a little more than 6 square inches of reflectance being added to the reflectance from the "bottom" of the sphere. The black tube does have a high negative impact, taking the first reading of 10400 down to 2880, a reduction on almost 73%. But, the percentage values from the first test are awfully close to those of the second test - the SF L4 having the largest test to test spread. Not a great surprise, with the wide angle, flood type beam dispersion of the L4.

I'll see if I can figure a way to mount both the lower & upper baffles.

Re: Building an Integrating Sphere ...

I just talked to my PhD Engineer coworker at lunch about the dual baffles. The only reason for multiple baffles is for units that have multiple input ports. Only one baffle is needed and used for a specific input location to the photodetector port. Our integration spheres only have 1 baffle. FWIW.

Re: Building an Integrating Sphere ...

divide all your readings by 36 (radius squared) to see how close they get to the rated lumens of the lights you use.

Even though the OD is 12", the ID is 10" (walls are 1" thick). My radius squared is 25 ... but the flat black tube to the light meter dome absorbs enough that my "corrected" radius squared becomes 12.5.

I've thought about this quite a bit, and plan to line the flat black tube with a bright white tube that has an angled cut end. As long as that angle is 45° or more, the long side of the tube will have the same effect as adding an internal baffle - direct light from the flashlight lens will strike the outside of the tube but not the inside where an inflated reading will result. At that point, the factor of 25 will probably be correct.

Re: Building an Integrating Sphere ...

I am confused??? Are you talking about the cut up maglight in the inlet port or is there something else on or around the actual light meter you use as your photodetector? I would have no built up walls to prevent light from coming in from all around the sphere to the detector except for the one baffle directly in line with the front end of a light at the inlet port. If you can't put it inside the sphere because the sphere is now sealed up, I would put it only at that limited angle along the opening of the inlet port that faces directly to the detector.

I also realized that your inside diameter was smaller than your outside one about 1 minute after I made that last post but I figured you would figure that part out real quick since you built it and would just see what you came up with anyway as you did, so I didn't bother to send a correction.

Re: Building an Integrating Sphere ...

I was able to remove the straight cut tube that points down from the sensor port (painted flat black inside) & replace it with a 45° angle cut tube painted flat white inside. The long point of the 45° cut faces the light port so that any spill will hit the point & be reflected. The opposite side of the tube is in the shadow of the 45° point, so no direct light can strike the inside of the tube. The reason for the flat white is to try to match the reflectance of the Styrofoam interior.

The output of the lights are still in the same order except that the U2 moves above the E1B. I played with the radius squared numbers and settled on 18 as being as close to published figures as possible. 18 = 4.24" radius, or about 8.5" internal sphere diameter. Best scientific guess is that the Styrofoam is not as bright as a commercial sphere, so my 10" is producing results like a commercial 8.5" sphere.

Latest (hopefully last) revision. Here are the readings. First reading is my lumen calculation, followed by factory stated lumens:

Re: Building an Integrating Sphere ...

That is starting to look really good. I have just received my Malkoff M60, I will measure it at work first chance I get next week. I already have measured the M60F at a peak of 202 lumens. I have the SureFire 6P, I can put it in that and will again measure it in the Solarforce L2 with 1 extension and the 2X17500 batteries that seem to work so well. If you were going to get one of these or can borrow one that is another reference.

If you have or can get a Fenix T1/TK10 I have measured those new at 225 lumens. Solarforce R2 single mode at 200 lumens max, no less than 180 lumens depending on how its mounted.

the mag light LEDs for the 4 c/d cell with no head on it but just sticking the exposed LED into the sphere until I got a peak reading were no better than 50 lumens. This would be a good test because all of the light is coming out the side of the emitter's special lens, so if your sphere works well, it would collect all that light and integrate it to give you 50 lumens.

I applaud your diligent effort to make it as accurate as possible and good for total comparative efforts.

Re: Building an Integrating Sphere ...

I applaud your diligent effort to make it as accurate as possible and good for total comparative efforts.

I appreciate you saying that. Your comments were my primary motivation to keep tweaking the design to get it as correct as possible.

Half-watt as caused me to test & retest the E1B ... it still gives readings that I consider extraordinary. But those readings are also supported by white wall shots & ceiling bounce meter readings - it is one bright light.

Using the correction or division factor of 18 produces lumen readings that closely correlate with most of the manufacturers stated outputs. I don't have any of the other lights you suggested testing, but I'll be happy to test any light that anyone sends, and I'll pay return shipping. I'd like to build as large a database as possible, but I currently have most every light that I want.

Re: Building an Integrating Sphere ...

Now that you have this data please put together this simple table of your measured Lumens readings versus the Vendor's stated Lumens. If its multimode list the mode (hi-med-low) on each individual line. Don't bother to list the lux readings as they are not useful in this quick comparison table. This will help to see how close your readings have come for those of us who are not familiar with some of those lights. As we build up the list of confirmed readings and the table grows longer we will see how good it is or if it needs further refinement.

But this really looks good.

I am very curious how it will track from the low level to the brighter lights and whether or not the Incans versus LEDs show any differences.

Since you have the 6P have you tried its normal incan light yet with good batteries? I think mine measured 50 lumens. Its another plot point to compare.

Even if you IS is not perfect, it will still be a much better total light measuring tool that will be effective for comparisons of various lights than other techniques. Readings from light to light, regardless of beam pattern should be more consistent for the various light types.

Now you should also consider building an exact duplicate and then testing that the readings are the same and figuring out the cost. Why you say? I am guessing that you will be able to sell it to one of these other guys who tests a boatload of flashlights and still have a boatload of flashlights available to test. If some one like Bessie Benny bought one and it was consistent and measured all those lights he had and got lumens readings to build a very large confirmed table set of readings, that would help everyone out.

You should give yourself a reasonable profit for your skills and labor that you put into it and don't let anyone make you feel bad about making an honest profit from your labor. You deserve it.

Of course if some of these guys are close to you they may want to come over and do a lot of testing with you and publish data out the wazoo.

Re: Building an Integrating Sphere ...

I measured the new Malkoff M60 in the IS at work today using the Surefire P6 bezel. It measured 220 lumens and dropped down to around 216 as it was warming up. I would use these numbers to calibrate the constant for your sphere. I talked to my PhD Optical Engineer coworker/friend today about your project. He did say that what ever the correction factor is, it would be a constant and work across the board. So if it winds up being 18 or 12 or XX we should get good readings using that constant for all readings of high and low power lights. Assuming for a moment that your Malkoff M60 is within 5% of what mine reads and I have a feeling it is if not within 2%, then I would take the lux readings off of that when it is dead center in the opening and just at the entrance, then divide by 216 and that should be your constant. I find that if I push the lights into the sphere the readings go down and if I pull them back away from the edge of the sphere the readings go down. tilting it doesn't have much effect which should be normal.

On a side note the M60F in the Solarforce deep crenalated bezel was down around 200 lumens. The bezel is blocking some of the light around the perimeter as it sticks out so far and the meter is sensing that small loss. So from now on all my pills will be read using the SureFire Bezel only.

I plan on sending you a Solarforce R2 5 mode pill to keep. I will take careful readings using 6 volts from 2 primaries as it doesn't like higher voltage, in the SureFire bezel, in high, med, and low mode. I will send you the readings and then send you the pill. Hang on to the pill as that will be your calibration reference unit when measured inside a SureFire Bezel with fresh primary batteries. May be a while before I can get this done, as I have some personal business to attend to but send me a PM with your mailing address if you want the pill.

If ever your readings start to drift, you can use the pill. If the time comes where we question the pill you can send it back and I can double check it and then send it back to you. Your lux meter may drift over time as they are all want to do.

Re: Building an Integrating Sphere ...

WOW!

I really appreciate all your effort. Work has kept me from replying to your first post, but I am also eager to get a 'calibration standard'. Industrial IS have a tungsten calibration light run from a constant voltage/constant current power supply.

whether or not the Incans versus LEDs show any differences.

I noticed that all the regulated LEDs stay at a near constant light meter reading. I did poke the 6P incan into the sphere & immediately noticed a reading that decreased a little bit every couple of seconds (non-regulated hotwire, I suppose). Each light meter does have an individual spectral response curve, so some meters will read some lights at a different value. My ExTech EA31 manual says "Calibrated to a standard incan lamp at a color temp of 2856K".

Assuming for a moment that your Malkoff M60 is within 5% of what mine reads

Readings are within 3% of each other I think my 18 number is awfully close to correct.

Re: Building an Integrating Sphere ...

The filaments of an incandescent lamp give off infrared energy. As the heat builds up the visible light energy drops off a little bit ever so slowly on incans. So the lumens readings does come down, beside what battery voltage drop is going to do.

I spoke to Gene Malkoff a while back. He doesn't actually have the equipment to measure lumens but goes by what some of his customers and vendors give him data on.

The M60 and F series LEDs, I am sure are capable of more than 235 lumens from the emitter, but with the optics in front of them, regardless of type there will be some loss of peak lumens.

I measured a real diffuser that I was given and it was dropping the lumens down by 10%. The 6 degree optic is great but I am sure that the output is not at the original 235 lumens. My reading of 220 I believe is much closer to the truth. Especially when compared side by side to the Fenix T1 by eye and by the IS, the Fenix is just a touch brighter and its starts at 230 and drifts down to 225. As the phosphor warms up its efficiency does drop every so slightly. This will stabilize and if you are holding steady for a longer ready than the first 10 seconds its usually gone.

Point is I think it would be good for now to assume your M60 is the same brightness as mine and use 220 lumens out to find your more finely tuned constant "correction factor number" and go with that.

I saw that you updated your previous set of numbers and it looks good. That one SureFire E1B measuring 130 lumens and rated at 80 is the only puzzling one. Are one of those numbers a typo? If not its always possible that what ever the controller components are supposed to be doing for regulation, it was set wrong or has a faulty component that it is allowing it to drive harder than what they had intended. Otherwise I would say that your baffle just isn't quite big enough and your positioning of the light is such that this one is still causing direct spill onto the photo detector some how.

by tilting the light at the input port the brightness should not really go up or down to any significant value since in theory the sphere is still collecting it all. If you find that your readings do change substantially, then the baffle is not large enough or in the exact best spot.

Of course if all other flashlights are reading true to form once we have the final fine tuning "calibration" I would say ignore this one light and be glad to have it (as I think you already are).

So I will try and take some readings of the 5 mode unit next week and send it out to you (provided you remember to pm with an address, but no rush).

Also do you have the original incan lamp for the P6 to measure with a fresh set of primary batteries?

and lastly, isn't it better to have a good feel for what the total light output is in lumens rather than a lux reading of the hot spot?

Once you know what 100 lumens really is, its easier to understand that the new Malkoff 60 puts out 220 plus, or the Fenix T1 puts out 225 or that a small EDC light puts out 55 lumens and know what that really means???

This is really good. I would expect that some people will want you to measure their lights as the data is getting very close to what I would call trustworthy.

Re: Building an Integrating Sphere ...

That one SureFire E1B measuring 130 lumens and rated at 80 is the only puzzling one.

Same here, G. White wall shots, side by side, as well as metered ceiling bounce tests (4' to ceiling + 4' bounce back) show the E1B as brighter than the 120P, and almost the same as the U2. Other members have made similar comments in PMs to me. It is awfully bright ... I may send it to you to test in your sphere at work.

tilting the light at the input port the brightness should not really go up or down to any significant value

Tilting makes no difference on any light ... there is a 'sweet spot' in the light tube that generates a max reading. Normally it's before the bezel of the light reaches the end of the tube, which projects just slightly beyond the inside of the foam wall.

Re: Building an Integrating Sphere ...

Very interesting discussion, I've been wanting to do this for some time. I own a 6" Hoffman IS, but it has a reference light source installed and I am reluctant to remove it for fear of losing the calibration.

Re: Building an Integrating Sphere ...

Good call on the link to plasteelcorp, that is exactly the one. Probably made by one foam molder & distributed by a dozen retailers. The 12" size works well, I'd try a larger one except that the larger sizes quickly increase in price.

it has a reference light source installed and I am reluctant to remove it for fear of losing the calibration.

That sounds like the wise approach. I really would like to figure out a light source of known luminous flux, then build a regulated DC power supply to drive that bulb - probably what you have in your Hoffman. But the numbers generated in my IS do track closely (in most cases) with those from the manufacturers. IMO, this is a much better approach than a one meter reading of the hotspot, which tells only a small part of the total story.